Colloids are disperse systems consisting of fine particles of the dispersoid and a continuous-phase dispersion medium; the dispersoid and the dispersion medium are not uniformly mixed. As a method for evaluating the form of this kind of non-uniform structure, electrical properties are measured, such as the conductivity, permittivity, etc. In recent years, in particular, methods for measuring permittivity using impedance measurements have been studied.
The Applicant proposed an electromagnetic induction-type conductivity and permittivity meter as an effective means for measuring the capacitance of solutions with large conductivities, i.e., their permittivities, in Japanese Patent Application No. 6[1994]-172023. This device solves the problem of errors due to electrode polarization, which is a disadvantage of the conventional electrode type, and is able to perform accurate measurements of permittivity without the effect of the conductivity. In that application, a probe structure and a simple correction method (calibration method) were proposed. The present invention is a proposal for improving the calibration method and reducing errors still further.
The permittivity measurement of the electromagnetic induction-type disclosed in Japanese Patent Application No. 6[1994]-172023 removes the effects of changes in conductivity of the solution on the measured permittivity value to a much greater extent, when compared to the electrode mode of measuring permittivity. This is because the electrode interfacial polarization is eliminated, principle. However, the following problem arises when the sensitivity multiplier (the so-called cell constant in the current equation) is corrected on the basis of the conductivity.
FIG. 2 shows an example of the expression of the true value and the measured value of the permittivity of an aqueous solution with respect to changes in frequency, using the conductivity of the solution as a parameter. The problem is that the measured value of the permittivity shows a lower value than the true one, and the measured value of the permittivity is affected by changes in conductivity. The magnitude of this error may be more than 30% in some cases. Moreover, when a solution is measured, in which the product of the permittivity and the angular frequency is almost equal to the conductivity in the measurement frequency range, the measured permittivity value is also changed by changes in frequency, as shown in the figure.
Before discussing the cause of this problem, we shall explain the electromagnetic induction-type probe of the prior art. FIGS. 6, 7, and 8 show first, second, and third actual examples, respectively, of the electromagnetic induction-type probe proposed in Japanese Patent Application No. 6[1994]-172023. Furthermore, the elements in each figure with the same functions are given the same numbers. These elements form the following structure. The main frame of impedance measuring instrument 1 has a signal source 2, a resistance 3, a voltmeter 4, and an ammeter 5.
Electromagnetic induction-type probe 8 has a primary core 10, a primary coil 11 and secondary core 12, a secondary coil 13, and a shield 14 with a gap 15; these elements are contained in an outer resin mold 9. Moreover, impedance measuring instrument 1 and probe 8 are connected by coaxial cables 6 and 7. Primary core 10 of the probe off FIG. 7 has a hole 16 running through it, and the probe of FIG. 8 has a balun 18 and a shunt 17. The equivalent circuit of the balun is shown by 19.
Simplified versions of the structures of FIGS. 6, 7 and 8 are as shown in FIG. 3. In this figure, the cross sections of the primary and secondary transformers and the inputs and outputs of the transformers are shown. When the excitation current supplied from signal source 2 flows to primary coil 11 and primary toroidal core 10 is excited, a concentric electrical field 31 is produced, the center of which is the center of the cross section of primary toroidal core 10. When the probe is dipped into the solution, the current surrounding the probe flows in the solution, due to electrical field 31, and as a result, secondary toroidal core 12 is excited and a current flows in secondary coil 13, producing a value on ammeter 5. From the vector ratio of this current value and the voltage applied to primary coil 11, which is measured by voltmeter 4, the conductance component and the susceptance component of the solution can be obtained.
Since the equivalent circuit of the solution can be expressed as parallel circuits of the resistance, which is determined by the conductivity of the solution, and the capacitance which is determined by the permittivity, method is performed by which the permittivity is obtained by a calculation process from the aforementioned susceptance component, i.e., capacitance component, of the solution.
FIG. 4 shows the current that flows due to the electrical field of the primary toroidal core. In the figure, cross sections of the primary and secondary transformers and the outer resin mold of the probe are drawn as constituent elements of the probe. The current that flows due to the electrical field of the primary toroidal core 10 includes: a current 32 that flows only in the solution, and a current 33 that flows from the solution, through the probe, and back into the current. Since the probe is filled with an insulator (not shown in the figure) which insulates the conductors, there is a stray capacitance (due to the dielectric effect of the insulator) in which a current flows. Furthermore, since the resistance value of the insulator is extremely high, the current that flows in the resistance component of the insulator can be ignored. The current 33 that flows in this stray capacitance is linked to secondary core. 12 and reduces the current that would be expected to flow. As a result, roughly speaking, the permittivity value measured is smaller than the true value.
As mentioned above, the fact that the measured permittivity value is lower than the true value and the measured permittivity value is affected by changes in the conductivity, are both caused by the stray capacitance in the probe.
Moreover, it is extremely difficult to make the value of this stray capacitance small enough so that its effects can be ignored, and doing so places many restrictions on the structure of the probe.